This work investigates the influence of coaxial air flow on droplet distribution, velocity, and size generated by a pressure-swirl atomizer. The experiments were performed inside a generic test section with large optical access at atmospheric conditions. The flow conditions replicate the mixing duct sections of high momentum jet stabilized combustors for gas turbines, e.g. high axial air velocities without swirl generation and high preheat temperatures.
High momentum jet stabilized combustors based on the FLOX® burner concept are used successfully in gas turbines due to its fuel and load flexibility and very low pollutant emissions. In previous and ongoing studies, different model combustors have been under investigation mainly with the focus of broadening fuel flexibility and operational limits. Operation with different liquid fuel injection systems in high pressure experiments showed a significant impact from the injector shape and injection strategy on the fuel air mixing behavior, the flame position and stability, and thus NOx emissions. This experiment will give a more detailed understanding of the turbulent mixing and interaction of primary and secondary atomization with the surrounding air in such burners. The setup will also allow for the testing of different injection systems for various burner configurations by the variation of injection type, location, fuel, and air flow properties.
In the present experiments a pressure-swirl atomizer was set to a constant pressure drop and mass flow. Liquid fuel was replaced by deionized water due to safety concerns. The coaxial air mass flow was preheated up to 473 K and set to bulk velocities of 20 m/s, 50 m/s, and 80 m/s. Particle Image Velocimetry (PIV) was used to characterize the flow field downstream of the point of injection. The droplet size and velocity distributions were quantified by double frame shadow imaging combined with a long-distance microscope with a resolution below 1 μm per pixel. Moreover, the formation of ligaments as well as primary spray break-up was visualized.
The results show a significant change of the spatial droplet distribution with increasing co-flow velocity for a given atomizer geometry. The spray cone angle widens at high co-flow velocities due to the formation of a pronounced recirculation zone behind the backward facing step of the injector near the nozzle orifice. This also leads to a change in the initial droplet momentum and the spatial distribution of large droplets. Smaller droplets are concentrated to the center of the spray due to turbulent transport.
These findings assist the ongoing developments of liquid fuel injection systems for high momentum jet based combustors and provide validation data for numerical simulations of primary and secondary atomization.